1. Field of the Invention
One or more embodiments of the invention relate generally to apparatus for analyzing liquids, such as body fluids, using labeled molecular affinity binding, such as immunochromatography. More particularly, the invention relates to strip test apparatus for detecting an analyte, such as an antibody or antigen, which may indicate a particular condition.
2. Description of Prior Art and Related Information
The following background information may present examples of specific aspects of the prior art (e.g., without limitation, approaches, facts, or common wisdom) that, while expected to be helpful to further educate the reader as to additional aspects of the prior art, is not to be construed as limiting the present invention, or any embodiments thereof, to anything stated or implied therein or inferred thereupon.
Labeled molecular affinity binding, such as immunochromatographic assays, have existed for decades and have proven to be an inexpensive way to screen for various conditions, such as abused drugs, and other conditions, such as pregnancy and cancer, or for single or multiple pathogenic conditions, such as HIV infection.
In the point-of-care test (POCT) setting, immunochromatographic assays are typically conducted using lateral flow strip technology as described in May et al., U.S. Pat. No. 5,656,503, incorporated herein by reference. With lateral flow devices, antibodies are movably supported on a solid support, such as a porous pad. Antigen derivatives are deposited as immobilized indicator lines downstream of the antibodies, whereby the target antigens in a fluid sample flow laterally as a liquid matrix by capillary action through the solid support. The antibodies are normally colored for visual indication. The fluid sample carries the antibodies downstream towards the indicator lines of immobilized antigen derivatives while a reaction takes place between the target antigens and the antibodies. Any antibodies that have not reacted with the antigen in the sample bind to the antigen derivatives at the indicator lines. When little or no target antigen is present in the sample, most or all of the colored antibodies are carried downstream to the indicator lines of the immobilized antigen derivatives. At the immobilized antigen derivatives, the colored antibodies bind together with the antigen derivatives in such concentrations that the colorant of the antibodies becomes readily visible. It is also known that the antigen derivatives' and the antibodies' roles can be interchanged. That is, the antigen derivatives can be labeled with colorant and movably placed in the solid support while the antibodies are placed as immobilized deposited indicator lines downstream.
Unfortunately, although they can be inexpensive and simple-to-use, depending on the type of condition being detected, these tests typically take from about 5 to 20 minutes to complete and provide a typical accuracy of between 75% and 95%, falling short of the 99% or above accuracy generally considered to be necessary for a confirmatory test. Moreover, these conventional tests provide no objective measure of a quantitative result, such as the concentration of a given drug present in the liquid being tested.
The reasons for the insufficient accuracy in many rapid in vitro diagnostic (IVD) test devices are primarily due to their current lack of overall higher sensitivity and specificity. Different samples may contain chemicals or particles which inhibit the rapid and well mixed liquid flow or otherwise interfere with one or both of the first and second affinity binding reactions.
Other prior devices have attempted to enhance sensitivity or specificity by pretreating various parts of the device with reaction or flow enhancing reagents, pH conditioning chemicals, or even non-specific adhesive blocking molecules which will “block-out” non-analyte molecules which might cause non-specific adhesion, or otherwise compete with the analyte in question for specific binding members, especially in the reaction zones region of the strip. These attempts have met with limited success in some types of testing, but do not provide the desired accuracy in many others. Also, pretreatment with two or more of the above pretreatments exacerbates the difficulties in obtaining uniform manufacturing due to potential incompatibilities between the pretreatment chemicals. For example, the pH conditioner might disrupt the effectiveness of the non-specific blocking member molecules. Moreover, the manufacturing step of pretreating with a second pretreatment chemical can dislodge some of the first pretreatment chemical.
Further, lot-to-lot variation in the manufacture of many IVD test devices can often lead to ambiguous results, such as false negatives as well as weak false positives, so-called “ghostlines” or “phantom lines”. False negatives typically occur when non-specific molecules interfere with the first and/or second affinity binding actions. It has been found that non-analyte molecules can clump together in liquid samples that are not well mixed so that they temporarily prevent access between analytes and binding members. Even temporary interference in past devices can prevent an adequate number of labeled analyte complexes and/or ultimately immuno-sandwich complexes from forming. In this way, if a non-analyte molecule or clump of molecules blocks access between analytes and binding members for only a few seconds, it may be enough to induce a false negative result. Further, clumps of non-analyte molecules can carry an overabundance of the labeled mobilizable binding members to the second affinity binding site to generate a false positive result.
Lateral flow devices are useful due to their low cost and ease of use. However, prior lateral flow devices suffer from low accuracy and relatively long test wait times as detailed above. This is especially true for saliva testing because of the low concentrations of analytes present. Current lateral flow strips cannot provide the necessary sensitivity and specificity within the time normally allotted to a typical law enforcement action such as a traffic stop.
The low accuracy can be due to a number of problems unique to lateral flow-type tests. First, there is often uneven movement of the immunoparticles within the nitrocellulose membrane. Smaller, non-analyte molecules mix together with the larger analyte molecules and compete for sites, often preventing the larger molecules from reacting in the desired fashion.
Therefore, there is a need to improve the accuracy of rapid IVD test devices so that rapid, inexpensive, easily conducted and quantitative immunological testing becomes a reality.